1. Field of the Invention
The present invention relates generally to a liquid immersed power transformer, and more specifically to an apparatus and a process of measuring a liquid level within a transformer without mechanical gauges.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Liquid-immersed power transformers, reactors and similar equipment are filled with a liquid to provide electrical insulation for, and transfer heat from, an energized internal component such as the core and windings, also referred to as the active part in the case of a transformer. The liquid can be mineral oil, synthetic oil, vegetable oil, or other liquids that provide the thermal and electrical performance requirements. The terms liquid or oil and the terms transformer or reactor have the same meaning.
The oil volume must be maintained at a level that is sufficient to cover all energized parts that need electrical insulation to prevent electrical failure, as well as a sufficient volume to transfer the heat energy produced by the active part to the radiators or heat exchangers for heat removal and maintaining the active part at safe operating temperatures where thermal damage will not occur.
Liquid-immersed power transformers have oil containment systems that can be classified in to two general categories that includes 1) gas-blanketed or 2) conservator type transformers equipped with oil expansion tanks. Both systems allow the oil level to rise and fall within a normal operating range with dynamic changes in operating and ambient temperatures.
While several transformer conditions can be monitored by measuring static and dynamic pressure, the only devices utilizing pressure measurement to-date have been electronic rapid pressure rise relays and gas pressure monitors for gas-blanketed transformers.
The pressure monitoring device covered under these specifications provides the ability to monitor liquid level, liquid volume, winding clamping condition, breather condition and to provide alarms on tank pressure and loss of liquid.
The current state of the art for oil level gauges is a mechanical gauge with a float mounted to a float arm. The top oil level in transformers with expansion tanks rises and falls within its normal operating range in an expansion tank as shown in FIG. 1. The top oil level in transformers with gas-blanketed designs rises and falls within its' normal operating range in the main transformer tank as shown in FIG. 2. In transformers with expansion tank designs, typical mechanical oil level gauges are mounted on the expansion tank with a long float arm inside the expansion tank that floats at the top level of the oil. In transformers with gas-blanketed designs, a typical mechanical gauge is mounted on the tank wall and utilizes an internal radial float arm that floats at the top level of the oil.
Mechanical gauges have a few drawbacks. They have a limited range of operation. The float arm can only move over a limited range so liquid levels below the minimum level or above the maximum level cannot be measured with a single gauge. Float arms can be bent or damaged easily. Float arms and floats can cause damage to flexible bladders that are often used inside expansion tanks to isolate the liquid from the outside air. Damaged flexible bladders can collapse inside the expansion tank and lay on top of the float arm, preventing the float from floating at the top level of the liquid and displaying a correct oil level. Mechanical gauges are constructed in two magnetically coupled parts and require gasketed seals between the internal float arm assembly and the outside gauge display assembly. These gasket seals degrade with time and temperature, and can be damaged by improper installation or maintenance, causing oil leaks and environmental problems. Mechanical gauges may be located in an area that is difficult to see due to interferences at the service location. Relocating mechanical gauges is not practical. The vast majority of mechanical oil level gauges are equipped with micro-switches to provide high and low level alarms, but the measured liquid level can only be determined by looking at the gauge, although some gauges have been developed recently with remote level indication. A mechanical gauge can only indicate liquid level, and cannot determine liquid volume because it does not measure or compensate for liquid temperature which can lead to improper filling when the transformer is filled with oil or similar problems.
Alternate pressure sensing designs have been proposed that utilize a pressure transducer on the bottom of the conservator tank and require temperature sensors to measure liquid temperature in the main tank. This type of system also has drawbacks including: the ability to measure liquid level is restricted to the area above the transducer so only the liquid level in the conservator tank can be measured; the system can only be used on conservator type transformers or reactors and not on gas blanket designs; the temperature of the liquid in the conservator tank must be estimated and extrapolated from the temperatures measured on the main tank which leads to low accuracy when calculating liquid density, specific gravity or thermal volume; the system requires direct thermal measurements for liquid level calculations; and, rapid detection of leaks is not possible.